Integrated direct air CO₂ capture and utilization via in-situ catalytic conversion to fuels and chemicals using dual functional materials: Recent progresses and perspectives

Abstract

Direct air capture (DAC) is an emerging technology aimed at mitigating global warming. However, conventional DAC technologies and the subsequent utilization processes are complex and energy-intensive. An integrated system of direct air capture and utilization (IDACU) via in-situ catalytic conversion to fuels and chemicals is a promising approach, although it remains in the early stages of development. This review examines the current technical routes of IDACU, including solid-based dual-functional materials (DFMs) through thermo-catalysis, IDACU using liquid sorbents with thermo-catalysis, and non-thermal conversion methods. It covers the basic principles, reaction conditions, main products, material types, and the existing problems and challenges associated with these technical routes. Additionally, it discusses the recent advancements in solid-based DFMs for IDACU, with particular attention to the differences in material characteristics between carbon capture from flue gases (ICCU) and DAC. While IDACU technology holds significant promise, it still faces numerous challenges, especially in the design of advanced materials.

Graphical abstract

Keywords

direct air capture (DAC) / integrated carbon capture and utilization (ICCU) / integrated direct air CO₂ capture and utilization (IDACU) / dual functional materials (DFMs) / in-situ catalytic conversion.

1 Introduction

Recent concerns about global warming, driven by greenhouse gas emissions, especially carbon dioxide (CO₂), have raised serious environmental issues that threaten human survival [1]. Over the past decade, the concentration of CO₂ in the atmosphere has been escalating at a rate of 2.4 μmol/mol annually, reaching an average of 420 μmol/mol in 2022 [2]. In response to this challenge, direct air capture (DAC) technology has emerged as a potential carbon-negative process to mitigate global warming, first proposed in 1999 [3], and gaining significant attention in recent years [4]. The captured CO₂ can be utilized in several ways, including enhanced oil recovery, chemical conversion, mineral carbonation [5,6], with catalytic conversion into high-value-added products such as chemicals or renewable fuels being considered the most promising approach [7].

Conventional DAC and CO₂ utilization processes operate separately via a gas processing system, often involving complex and energy-intensive steps such as compression, storage, and transportation [8]. To overcome these limitations, a more efficient alternative is the integration of DAC and CO₂ conversion without the need for CO₂ desorption. This concept can be divided into two main configurations. The first involves coupling a DAC reactor with a separate (thermal [9,10] or electrochemical [11]) conversion reactor, as shown in Fig. 1 [9]. In this setup, sorbents containing captured CO₂ are transported to the conversion reactor for CO₂ transformation, after which the sorbents are regenerated and returned to the DAC reactor. This configuration avoids time-consuming and costly processes of CO₂ desorption and transportation.

A more integrated approach is the direct air CO₂ capture and utilization (IDACU) system, where DAC and in-situ catalytic conversion occur in the same reactor. This configuration uses dual-functional materials (DFMs), as shown in Fig. 2 [12]. In this system, the captured CO₂ is directly converted into products like methane [12–21], methanol [22–24], formic acid [25–27], carbon monoxide [28–30], dimethyl carbonate (DMC) [31], or cyclic carbonate [32,33] through thermal or non-thermal catalysis methods. The need for regeneration, which significantly contributes to the energy consumption of DAC cycles [4], is eliminated in this process. In addition, the desorbed CO₂ required for catalytic conversion must be of high purity, and the associated regeneration and purification steps can be energy-intensive [34]. Therefore, the IDACU system, which integrates DAC with in-situ conversion to chemicals or renewable fuels, has great potential to reduce energy consumption, simplify the process, and serve as an attractive carbon-neutral technology to reduce carbon emissions and store renewable energy. This review focuses on the integrated DAC and in-situ conversion system, or IDACU.

Integrated carbon capture and utilization (ICCU) technology based on DFMs for flue gases from power plants and industrial sources (with CO₂ concentrations ranging from 5% to 30%) has been widely investigated since its introduction by Duyar et al. [35] in 2015. Numerous reviews have focused on materials, mechanisms, processes, techno-economic analysis, and industrial application prospects of ICCU [36–40]. As the carbon source shifts from flue gases to air, ICCU technology can evolve into IDACU. However, the selection of materials for IDACU is different due to the vast contrasts between atmosphere air and flue gases, not only in CO₂ concentration but also in temperature. For example, DFMs for IDACU must consider the adsorption kinetics caused by the very low CO₂ concentration (around 400 μmol/mol). In addition, while ICCU for flue gases generally operates at high temperatures (around 300–800 °C) [41,42], most DFMs in this field are based on solid sorbents. In contrast, IDACU processes are generally preferred to operate at atmospheric temperature to avoid the high energy consumption associated with heating large volumes of air for a very long period of time. As a result, both solid sorbents and liquid solutions can be applied to IDACU. Although extensive research has been conducted on ICCU for flue gases, IDACU still faces many challenges and has great potential for development.

Two key reviews on DAC and utilization of CO₂ from DAC provide comprehensive technical routes for CO₂ capture and utilization [2,5]. However, research and corresponding reviews on the integration of DAC and in-situ catalytic conversion are relatively scarce, mainly due the novelty of this approach. This review focuses on the integration of DAC with in-situ catalytic conversion to chemicals and renewable fuels, especially in systems where the captured CO₂ is not released. First, it reviews the existing technical routes of IDACU, categorized into solid-based DFMs via thermo-catalysis, IDACU using liquid sorbents with thermo-catalysis, and non-thermal conversion methods. Then, it provides an in-depth discussion of the recent advances in solid-based DFMs for IDACU, with special attention to the differences in material characteristics between ICCU of flue gases and DAC.

2 Technical route of integrated DAC and in-situ catalytic conversion

IDACU can be implemented through different technical routes, including solid-based DFMs via thermo-catalysis [12–19,31,32], liquid sorbents through thermo-catalysis [20–24,33,43–46], and non-thermal conversion processes [25–30]. This section outlines the basic principles, reaction conditions, main products, types of materials, and the challenges associated with these technical routes. Themo-catalysis is a relatively mature and widely used method. Among them, the separation, regeneration, and recycling of materials are issues that require special attention.

Thermo-catalysis is a well-established and widely used method in IDACU. Key issues in this approach include the separation, regeneration, and recycling of materials. Based on the material systems involved, thermo-catalysis can be further classified into solid-based DFMs [12–19,31,32] and liquid sorbents [20–24,33,43,46]. Liquid sorbents can be further divided into heterogeneous catalytic systems [20,21,24,46], where the liquid absorbent is combined with a solid catalyst, and homogeneous catalytic systems [22,23,43], where both the adsorbent and catalyst are liquid materials.

In solid-based DFMs, the sorbents and catalysts are integrated into a single solid material system. After completing one IDACU cycle, there is no need to separate the adsorbents and catalysts, as they can be directly used for the next capture cycle. Heterogeneous catalytic systems only require solid–liquid separation for material recycling, while homogeneous catalytic systems require more complex processes, such as distillation and other methods, to regenerate the liquid adsorbent and catalyst. Therefore, the ease of material recycling for the three methods can be ranked from easiest to most difficult as solid-based DFMs < heterogeneous catalytic systems < homogeneous catalytic systems.

However, thermo-catalysis is often limited by high energy consumption and conversion efficiency [12–24,31,32,43–46]. To address these limitations, novel non-thermal conversion technologies, such as reactions with other reactants [25,26], photocatalysis [28,29], electrocatalysis [27] and photoelectrocatalysis [30], are being explored for IDACU. The main technical routes, along with their respective advantages and disadvantages, are summarized in Fig. 3.

2.1 Solid-based DFMs through thermo-catalysis

A typical solid-based DFM used in the IDACU applications is composed of an adsorbent and catalyst, and, in some specific cases, a support and promoter, although these are not always necessary. As shown in Fig. 4 [13], IDACU operates in two main steps: during the capture phase, the DFM absorbs CO₂ from the air; during the conversion phase, the captured CO₂ reacts with H₂ or other reactants under the influence of catalyst sites, forming value-added products such as CH₄ [12–19], DMC [31], and cyclic carbonates [32].

DFMs can usually be divided into supported and unsupported types, with room for significant improvements in both CO₂ capture and conversion performance. Two primary temperature-operation methods are considered: temperature-swing operation and isothermal operation, which differ in their capture temperatures. In temperature-swing operation, the DAC stage occurs at ambient temperature (20–40 °C), while the conversion stage is heated to a higher temperature (e.g., > 300 °C for methanation). In contrast, isothermal operation involves maintaining the same temperature for both capture and conversion stages. The temperature-swing method is generally considered more cost-effective because it avoids the high energy consumption associated with heating large volumes of air and reduces the risk of catalyst oxidation due to exposure to oxygen at elevated temperatures [12,15].

Current studies on IDACU using solid-based DFMs with temperature-swing operation strategies are mainly focused on the in-situ methanation pathway. In 2018, Veselovskaya et al. [14] were the first to achieve integrated DAC and methanation. They used a 4%Ru/Al₂O₃ catalyst and a 22.1%K₂CO₃/Al₂O₃ sorbent, placed in a segmented single reactor, to realize DAC at ambient conditions and methanation at 300/350 °C. After that, Jeong-Potter et al. [15] synthesized a Ru + Na₂O/Al₂O₃ DFM using the incipient wetness method, achieving a CO₂ capture capacity of 1.3 mmol/g at 25 °C and 90% relative humidity (RH) in the air, as well as an 80% CO₂ conversion at 300 °C. It is worth noting that they found that the relative humidity of air had a significant effect on both DAC and methanation performance, with CO₂ capture capacity and CH₄ yield increasing 2.36 and 3.47 times, respectively, compared to the dry conditions. They also compared the performance of Ru + CaO/Al₂O₃ DFM, which, although lacking in capture capacity and cycling performance, partially addressed the CO₂ desorption issue during the conversion stage.

Based on this research, Jeong-Potter’s group evaluated a 0.25%Ru, 6.1%Na₂O/γ-Al₂O₃//monolith for approximately 250 h on stream at ambient conditions in DAC, followed by methanation at 280 °C, which exhibited good stability and demonstrated the potential for large-scale IDACU applications [16]. Lee’s group proposed a 5%Ru/40%K₂CO₃/K-β” Al₂O₃ DFM, which showed a CO₂ capture capacity of 0.170−0.251 mmol/g at 30 °C and a CH₄ yield of 0.113−0.161 mmol/g at 300 °C. They compared two supports, K-β” Al₂O₃ and γ-Al₂O₃, and found that K-β” Al₂O₃ had a lower methanation temperature, better CH₄ selectivity, and improved cyclic stability due to the positive effect on the CO₂ activation and the increase of the methanation reaction sites introduced by K⁺ [17].

However, a limitation of the DFMs in the studies above is that both the adsorbent and catalyst of the DFMs are supported, leading to the unavoidable problem of a CO₂ capture capacity lower than the theoretically calculated value of the sorbent. Besides, the use of noble metal Ru catalysts raises concerns about high costs, despite the relatively low methanation temperatures. To address these issues, Feng et al. [12] proposed novel unsupported NiCa-based DFMs, with Ca(OH)₂ as the sorbent and Ni as the catalyst, which demonstrated an outstanding performance in temperature-swing tests, with DAC at 25 °C and methanation at 450 °C, achieving a CO₂ capture capacity greater than 7 mmol/g and CO₂ conversion greater than 95%. The performance of these DFMs is summarized in Table 1 [12].

There were two primary technological pathways for IDACU applications with temperature-swing operation. Kosaka et al. [18] prepared a Ni/Na-γ-Al₂O₃ DFM and tested the IDACU-methanation under both temperature-swing operation (DAC at room temperature and methanation at 450 °C) and isothermal operation (450 °C). They found that lower capture temperatures help inhibit the release of unreacted CO₂. Jeong-Potter et al. [19] tested the performance of a 0.5%Ru, 6.1%“Na₂O”/Al₂O₃ DFM, which showed a CO₂ capture capacity of approximately 0.2 mmol/g and a CH₄ yield of approximately 0.15 mmol/g at 320 °C. Another pathway is the esterification. Wotzka et al. [31] proposed a Ce_0.8Zr_0.2O₂ solid solution for integrated DAC and subsequent conversion to DMC by reacting with methanol under mild conditions (1 bar, 70−110 °C) with an isothermal operation. The CO₂ capture capacity ranged 0.032 to 0.076mmol/g, and the DMC yield ranged from 1.4% to 74.9%. Besides, a novel Mg(II)-based MOF was proposed by Das et al. [32], as an effective catalyst for the direct fixation of CO₂ from air into cyclic carbonates under mild conditions (60 °C), achieving an epichlorohydrin conversion rate of 92%.

2.2 Liquid sorbent through thermo-catalysis

CO₂ is captured by liquid sorbents and then converted into other products via a thermo-catalytic process. First, CO₂-containing air is passed into the adsorbent, which becomes saturated after a long period of adsorption (carbon capture process). Then, a reducing agent or other reactant is introduced to convert the captured CO₂ into fuels or chemicals (catalytic conversion process). To integrate capture and conversion, either the sorbent is transferred, or the air path is switched.

The liquid sorbent usually used include amine solutions (such as PEHA) [22,46], alkaline hydroxide solution (NaOH, KOH) [20,21,23,24,43], or ionic liquid (IL) [33]. The catalysts can be divided into two categories: homogeneous [22,23,43] and heterogeneous [20,21,24,46]. Homogeneous catalytic systems are more suited for producing liquid products, such as methanol and formate, due to their higher carbon capture capacity and mass transfer rates in liquid systems. In contrast, gaseous products such as CH₄ are usually produced in heterogeneous catalytic systems [20,21].

However, one challenge with homogeneous catalytic systems is costly separation of the catalyst and adsorbent. Although heterogeneous catalytic systems do not face this issue [47], achieving a continuous cycle of capture and conversion remains a challenge. It is worth noting in particular that the terms “homogeneous” and “heterogeneous” catalysis here are defined based on the phase states of the catalyst and adsorbent, rather than the traditional definitions of homogeneous and heterogeneous catalysis in the broader field of catalysis.

2.2.1 Heterogeneous catalytic system

When the catalyst and adsorbent are in different phases, they form a heterogeneous catalytic system. Generally, a solid catalyst is added to the sorbent solution to form a heterogeneous catalytic system. Liquid products can also be produced in such systems. For instance, Ni et al. [46] used a pentaethylenehexamine (PEHA) solution as the sorbent and Au/SiO₂ as the catalyst in a heterogeneous catalytic system to hydrogenate CO₂ to C1 products (formate, formamide, and methanol). The reaction was conducted at 100 °C, 16 bar for 48 h, achieving a yield of 80%. Similarly, Koch et al. [20] proposed a heterogeneous catalytic system with KOH solution as the sorbent and 5%Ru/Al₂O₃ as the noble metal catalyst, in which CO₂ from the air was captured in K₂CO₃ and then reduced by H₂ to generate CH₄ at 250 °C and 60 bar for 24 h.

In heterogeneous catalytic systems, non-noble metals can also be used as catalysts to reduce costs. For example, Raktim et al. [24] utilized non-noble Cu/ZnO/Al₂O₃ as the catalyst, combined with alkali hydroxides and amines as sorbents, achieving 90% CO₂ hydrogenation to methanol for the first time. Besides, glycols were found to significantly enhance CO₂ hydrogenation to methanol within a low-temperature range of 170−200 °C. This led to the concept of solvent-assisted integrated CO₂ capture and conversion to methanol (Fig. 5). In 2024, Sen et al. [21] introduced a lanthanide-promoted non-noble metal catalyst, 50%Ni/12.5%Yb/Al₂O₃, to replace the Ru/Al₂O₃ catalyst, while still using KOH solution as the sorbent, which demonstrated a high CH₄ yield of 100% under milder conversion conditions (225 °C, 50 bar for 24 h).

2.2.2 Homogeneous catalytic system

A homogeneous catalytic system is formed when the catalyst and adsorbent are in the same phase, allowing for more intimate contact between the two and thus improving the mass transfer process. Professor Prakash’s group at the University of Southern California has extensively studied carbon capture and homogeneous catalysis. In 2015, they successfully generated methanol using a PEHA liquid as the adsorbent and Ru-Macho-BH as the catalyst at 155 °C and 50 bar, achieving a yield of 79% [22]. Furthermore, the methanol was separated through simple distillation, marking the first demonstration of the feasibility of direct air carbon capture and hydrogenation to methanol in a homogeneous catalytic system [22].

On this basis, the cyclic carbon capture and conversion performance of homogeneous catalytic systems was further investigated. One major challenge is the separation of the homogeneous catalyst from the adsorbent before the next cycle. Another important consideration in cyclic homogeneous catalytic systems is the regeneration of the sorbent. While amine solutions are commonly used as sorbents, alkaline solutions (such as KOH and NaOH) have also been adopted. The main issue with alkaline sorbents is the regeneration of hydroxide.

To simplify the process, a new strategy has been proposed to eliminate the need for separating the catalyst in homogeneous catalytic systems. Using a fuel cell, the catalytic reaction products can be directly converted into electrical energy [43]. In this approach, the captured CO₂ is converted into formate salts at 80 °C and 50 bar using an alkaline solution (NaOH and KOH) as the sorbent and Ru- and Fe-based PNP complexes as catalysts. The formate aqueous solution generated does not require any purification and can be used directly in formate fuel cells for power generation and alkali hydroxide regeneration, thus realizing a carbon-neutral cycle.

To further simplify the regeneration process, Sen et al. [23] introduced the first concept of a KOH-based system for integrated DAC and in-situ conversion into methanol (Fig. 6). In this system, CO₂ from the air is captured by a KOH solution in ethylene glycol to form carbonates and alkyl carbonate salts for 48 h. These carbonates are then hydrogenated to methanol with a CH₃OH yield of 100% after 72 h under the effects of Ru-PNP catalysts. The KOH solution and ethylene glycol sorbent can be regenerated during the conversion stage at a relatively low temperature (140 °C).

2.2.3 Other catalytic system

Typically, organic amines are used in solution as sorbents. However, solid organic amines can also be used directly as adsorbents. Studies have also explored heterogeneous catalytic systems consisting of solid organic amines and liquid catalysts [44]. This approach provides a practical solution to the challenge of separating the solid amine from the system after the reaction. However, this heterogeneous catalytic system differs from those previously discussed. In this case, the solid amine must be loaded on other supports, and there is a risk of leaching as the reaction temperature increases. This can lead to partial dissolution of the adsorbent in the liquid catalyst, resulting in a catalytic system that contains both heterogeneous and homogeneous phases.

Metal-based catalysts are commonly used in both homogeneous and heterogeneous systems to facilitate or improve the conversion of captured CO₂. However, the addition of a catalyst increases the overall cost of IDACU applications, especially when noble metal catalysts are used. Recently, Zanatta et al. [33] proposed an IDACU system that operates without catalysts offering a more economical approach. In this system, commercially available hydroxide-based ionic liquids (ILs), such as TBA.OH with DMSO or DMC solvents, are employed as sorbents to capture CO₂ from the air as TBA.HCO₃, which is further reacted with epoxides and halohydrins to generate the cyclic carbonates with 100% conversion and selectivity without catalyst under mild conditions (60 °C). As shown in Fig. 7, after the conversion stage, the sorbents are converted into TBA.X and mixed with the product. To obtain the final product and reuse the sorbent, the mixture undergoes a washing process to separate the product from ILs. An ion exchange resin method is then utilized to reactivate the TBA.OH sorbent.

In summary, when the catalyst and adsorbent are in a homogeneous phase, the product is usually in a liquid state, which benefits from the higher carbon capture capacity and better mass transfer characteristics of the liquid-phase system. However, it is challenging to separate the products in the liquid system, often requiring distillation. Moreover, the reaction usually takes place under high-pressure conditions, and in some cases, even high temperatures, which imposes greater demands on the stability of both the adsorbent and the catalyst. It is worth mentioning that the systems described above, where the catalyst and adsorbent are in separate materials, do not qualify as dual-function materials (DFMs) in ICCU. This is because DFMs integrate both the adsorption and catalytic functions into a single material, while in the systems discussed, the catalyst and adsorbent remain distinct, and the liquid-phase system is more complex than simply mixing the two components.

2.3 Non-thermal conversion processes

The solution system faces severe challenges at high temperatures during catalytic conversion. Therefore, many researchers have shifted their focus toward non-thermal catalytic systems, which can perform the catalytic reaction process at lower temperatures, or even at room temperature, using methods like photocatalysis. Lombardo’s group [25,26] has explored the use of ILs containing BH₄⁻ to capture CO₂ from the air at room temperature. They also introduced HCl at room temperature to facilitate the conversion, resulting in the production of formic acid, as shown in Fig. 8. The carbon capture capacity of the IL is notable, reaching 1 g CO₂/g[EMPY][BH4], due to the binding of one BH₄⁻ ion to three CO₂ molecules. However, the high carbon capture capacity leads to long adsorption times to reach saturation, especially in air carbon capture, which undoubtedly causes great inconvenience for continuous operation. Nonetheless, the technology is attractive because it eliminates the need for using expensive metal catalysts and toxic solvents such as DMF and DMSO.

Photocatalysis is another important non-thermal catalytic route to achieve catalytic conversion of CO₂. Fan et al. [28] used a carbonate-type CuCoAl-layered double hydroxide (LDH) to both concentrate and catalytically convert CO₂, producing a wide range of products, including reduction product like CO and oxidation products such as 2.5-furandiformaldehyde (DFF), 5-formyl-2-furanacarboxylic acid (FFCA), and 2.5-furan dicarboxylic acid (FDCA), as shown in Fig. 9. The temperature during the catalytic conversion process was maintained at 34‒37 °C. Interestingly, the depleted CO₃^2‒ could be easily replenished by CO₂ from the air, allowing CO₂ adsorption and catalytic conversion to occur simultaneously, which provides a more convenient and continuous pathway for carbon capture and conversion. In addition, photocatalysis can be integrated with membrane separation technology for enhanced CO₂ utilization. In this setup, a selective Janus membrane was attached to one side of a Cu/TiO₂ photocatalyst. The Janus membrane selectively allowed CO₂ to permeate while blocking other components [29]. The permeated CO₂ was then reduced to CO via catalysis. The technology eliminates the need for a separate carbon capture process, combining both CO₂ enrichment and catalytic conversion processes in a single step.

Cobb et al. [27] proposed an innovative strategy to directly reduce CO₂ from atmospheric concentrations to formate using electrochemical systems with biological catalysts. They adapted a nanoconfined carboxysome-inspired system to accelerate CO₂ hydration kinetics, ensuring that all dissolved carbon is efficiently utilized. This system enables CO₂ to be cleanly reduced to formate with the aid of a highly efficient formate dehydrogenase.

Kar et al. [30] proposed a novel integrated photoelectrochemical (PEC) system capable of capturing CO₂ from the air and directly converting it into syngas (with V(CO) : V(H₂) = 1:30), using sunlight as the sole energy input. The proposed concept and reaction processes, illustrated in Fig. 10, involve a two-step procedure. First, indoor air is pumped through a capture solution (1 mol/L aqueous TEA or glycolic NaOH) for 2 days using an aquarium pump. Then, the captured CO₂ is reduced to syngas in a perovskite-based photocathode of the solar-driven PEC system, aided by an immobilized molecular Co-phthalocyanine (CoPcNH₂@MWCNT) catalyst. Simultaneously, the plastic-derived ethylene glycol at the anode is oxidized to glycolic acid at the anode using a Cu₂₆Pd₇₄ alloy catalyst. The study compared the effectiveness of aqueous TEA and glycolic NaOH solutions as DAC adsorbents for CO₂ capture and conversion. The results showed that the glycolic NaOH performed better in the solar-driven integrated system, with a higher CO₂ uptake of 0.73 ± 0.07 mmol per mol NaOH and a total CO production of 2.1 ± 0.5 μmol cm⁻² after 110 h. In contrast, when aqueous TEA was used as the adsorbent, only 0.02 ± 0.01 mmol CO₂ per mol TEA was captured and no CO was produced. This technology not only converts CO₂ from the air into a value-added product but also facilitates waste recycling, powered solely by sunlight at ambient temperature and pressure.

In conclusion, non-thermal catalysis has particularly unique advantages over conventional thermal catalytic conversion, including reduced thermal stability requirements, elimination of external heat sources, and the potential for simultaneous CO₂ capture (or enrichment) and catalytic conversion. These novel technological routes provide a new perspective on the resource utilization of CO₂.

3 Advances in DFMs for IDACU

3.1 Supported DFMs

In IDACU DFMs, commonly used sorbents include oxides or carbonates based on alkali and alkaline earth metals, such as Na₂O (Na₂CO₃) [13,15,16], K₂CO₃ [14], and CaO [15]. The typical catalysts used are both noble metals like Ru [13,15–17] and non-noble metal such as Ni [12]. Due to the ultra-low CO₂ concentration in the air, capturing CO₂ directly requires specialized materials with unique microstructures. The supports used in these DFMs are designed to have a large specific surface area, special pore structures, and high thermal stability, which is conducive to the uniform dispersion of both sorbents and catalysts. This improves the adsorption, conversion, and cycling performance of the DFMs. The main supports used in ICCU applications include Al₂O₃, CeO₂, SiO₂, TiO₂, MOF. Among these, Al₂O₃ is the most used support in IDACU studies to date [13,15–17]. In IDACU applications, supported DFMs are synthesized using the impregnation method, where the sorbent is first impregnated onto the support, followed by the catalyst. This method usually involves the impregnation, drying, and calcination steps [13,15–17].

In the DAC stage, different sorbents such as Na₂CO₃ [15], K₂CO₃ [17], and CaO [15] exhibit distinct carbonation reaction mechanisms, as shown in Eqs. (1)−(3). After saturated absorption, the captured CO₂, in the form of carbonates, can react with H₂ to generate CH₄, as described in Eqs. (4)−(6). Lee et al. [17] applied the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technology to observe the intermediates formed in the integrated DAC and in-situ methanation process on Ru/K₂CO₃/K-β” Al₂O₃ and Ru/K₂CO₃/γ-Al₂O₃. Based on these observations, they proposed a possible reaction mechanism. During the methanation step, formate (HCOO*) and carbonyl (CO*) were identified as the two main intermediates [17].

(1){\mathrm{N}\mathrm{a}}_2{\mathrm{C}\mathrm{O}}_3+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{C}\mathrm{O}}_3

(2){\mathrm{K}}_2{\mathrm{C}\mathrm{O}}_3+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{K}\mathrm{H}\mathrm{C}\mathrm{O}}_3

(3)\mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{C}\mathrm{O}}_2\to {\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3

(4)2{\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{C}\mathrm{O}}_3+4{\mathrm{H}}_2\to {\mathrm{N}\mathrm{a}}_2{\mathrm{C}\mathrm{O}}_3+{\mathrm{C}\mathrm{H}}_4+3{\mathrm{H}}_2\mathrm{O}

(5)2{\mathrm{K}\mathrm{H}\mathrm{C}\mathrm{O}}_3+4{\mathrm{H}}_2\to {\mathrm{K}}_2{\mathrm{C}\mathrm{O}}_3+{\mathrm{C}\mathrm{H}}_4+3{\mathrm{H}}_2\mathrm{O}

(6){\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3+4{\mathrm{H}}_2\to \mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{C}\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O}

3.2 Unsupported DFMs

Although the supports offer some benefits advantages, the supported DFMs still face significant challenges. The CO₂ capture capacity of these materials is much lower than the theoretically calculated value of the sorbent due to the relatively low sorbent content (6.1%–40%) [13,15–17]. Besides, the DFMs based on Na and K sorbent suffer from unreacted CO₂ release during the heating and methanation stage, because of their relatively weak basicity [15,17]. While CaO sorbents can decrease the amount of CO₂ released, they are considered medium- to high-temperature CO₂ sorbents (> 600 °C) [48], which limits their CO₂ capture capacity and rate at ambient temperature. To solve these address these issues, Feng et al. [12] proposed novel NiCa-based DFMs, incorporating Ca(OH)₂ as the sorbent, Ni as the catalyst, and metal oxides with high Tammann temperatures (such as MgO, CeO₂, and ZrO₂) as promoters. The use of Ca(OH)₂ as a DAC sorbent was first proposed and achieved by Samari’s group in 2020 [49]. They showed that Ca(OH)₂ could effectively capture CO₂ from air at room temperature in the presence of H₂O (Eq. (7)). Besides, Ca(OH)₂ has strong basic sites that enhance both CO₂ capture and conversion. The CO₂-temperature programmed desorption (TPD) tests further confirmed that no CO₂ was released below 500 °C. Therefore, the NiCa-based DFMs could effectively prevent CO₂ release and achieve high CO₂ conversion. In addition, the authors of the present paper employed the sol-gel method to prepare the unsupported DFMs, which resulted in highly uniform dispersion of compositions and ensured that the sorbent dominated the composition, thereby increasing the CO₂ capture capacity per unit mass of the DFMs [12].

(7)\mathrm{C}\mathrm{a}(\mathrm{O}\mathrm{H}{)}_2+{\mathrm{C}\mathrm{O}}_2+n{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3+(n+1){\mathrm{H}}_2\mathrm{O},

A possible reaction mechanism for integrated DAC and in-situ methanation on NiCaZr DFM is proposed, as shown in Fig. 11. During the DAC stage, CO₂ and H₂O are adsorbed on the DFM surface, forming carbonates. Meanwhile, the formed CaCO₃ can further react with CO₂ and excess H₂O from the air to generate bicarbonates, which then react with unreacted Ca(OH)₂ to produce additional carbonates. This process enhances both the CO₂ capture capacity and rate. During the methanation stage, H₂ dissociates into H atoms under the influence of Ni. These H atoms interact with the carbonates in the DFM to form bicarbonates, which are subsequently hydrogenated and dehydrated to produce formates (*HCOO⁻). There are two main methanation pathways: 1) the formates on the adsorbent surface are directly hydrogenated to CH₄, and 2) formates on the Ni surface are first dehydrated to form carbonyl (*CO(Ni)), which is then hydrogenated to CH₄ [12].

3.3 Other types of DFMs

New types of DMFs such as solid-supported amine combined with catalyst in solution, membrane materials, and composite oxides have been developed. Kar et al. [44] developed a novel adsorption-desorption technique, as shown in Fig. 12. In this method, organic amines such as PEI and PEHA were chemically fixed onto a hydrophilic silica solid support using cross-linking agents like glyoxal to improve CO₂ adsorption performance and inhibit the leaching of organic amines. Catalysts such as Ru-Macho-BH, RuHClPNPPh(CO), RuHClPNPiPr(CO), and MnBrPNPiPr(CO)₂ were used in the reaction, which was conducted in a reactor at 145 °C under pressurized hydrogen. Since the organic amines are fixed on the solid support, they can be easily recovered after the reaction, facilitating reuse. However, despite this advantage, the persistent leaching effect of organic amines, negatively impacts the long-cycle capacity retention of the material, indicating that further development is required to optimize this technique.

Li et al. [45] developed a system that first uses a membrane to concentrate CO₂ from 400−500 to 2000 μmol/mol. The membrane material used is a PDMS/PAN bifunctional material. Once purified, the gas passes through this bifunctional material, significantly enhancing the adsorption rate. The bifunctional material consists of Ni-Ca/Al₂O₃ with 10 wt.% Ni and 6 wt.% CaO. Wotzka et al. [31] explored a Ce_0.8Zr_0.2O₂ material that first adsorbs CO₂, and then reacts with methanol to produce DMC. The reaction occurs at a low temperature range of 70−110 °C, which makes it promising for practical applications. By using a high concentration of methanol precursor (over 33.4 vol%), the material can achieve a 71% CO₂ conversion rate. However, the material’s CO₂ adsorption capacity (0.104 mmol/g), the product concentration, and the desorption process, which takes 30 h to complete a full desorption cycle, are relatively low, limiting its practical application.

4 Conclusions and outlook

DAC technology has emerged as a potential carbon-negative process. However, conventional DAC and utilization processes are typically operated separately, requiring complex and energy-intensive procedures for compression, storage, and transportation. To address these challenges, integrating DAC with in-situ catalytic conversion to fuels and chemicals (IDACU) is a promising approach. Nonetheless, IDACU remains a very early-stage field of research and faces several obstacles. This review examines the existing technical routes of IDACU, including solid DFMs via thermo-catalysis, IDACU using liquid sorbents with thermo-catalysis, and non-thermal conversion technologies. The review covers the basic principles, reaction conditions, main products, types of materials, and the current challenges associated with each technical route. The design of advanced materials plays a crucial role in the development and practical application of IDACU, especially in the thermo-catalysis route. Additionally, it summarizes the recent advancements in supported DFMs, unsupported DFMs, focusing on material formulas, adsorption and conversion performances, mechanisms, and material characteristics. Beyond material properties, factors such as large-scale production of the material, structural design of air contactor, and cost and energy consumption are vital for the successful future application of IDACU technology.

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